Characterizing Ice Nucleating Particles Emitted from Agricultural Activities and Natural Landscapes

Soil dust and plant fragment emissions from agricultural harvesting and natural ecosystems are two potentially large, yet unquantified and largely uncharacterized, sources of ice nucleating particles (INPs). Both organic and mineral components have been shown to contribute to the ice-nucleating ability of soil dust, but apart from the likely presence of ice nucleation-active bacteria, little is known about the ice nucleating potential of plant tissues. This work aims to identify and differentiate the organic and inorganic contributions of soil and plant INP sources emitted from harvesting activities and natural landscapes. For this purpose, the CSU Continuous Flow Diffusion Chamber (CFDC) and the Ice Spectrometer (IS) were utilized in a combination of ambient measurements and laboratory studies. Small variability and low INP numbers (< 10 L-1 at -30 °C) characterized measurements made in air over the grazed Pawnee National Grassland in Colorado, while more variable INP over croplands around the DOE-ARM SGP site in Oklahoma appear linked to regional wind, humidity, and rainfall conditions. Harvesting of milo (grain sorghum), soybean, and wheat at an experimental research farm in Kansas resulted in spikes of INPs, with wheat harvesting producing the largest INP concentrations (up to 100 L-1 at -30 °C). In-situ use of heating tubes upstream of the CFDC to deactivate organic INP showed that milo and wheat harvest emissions showed a stronger reduction of INPs at warm temperatures than soybean emissions, suggesting a larger contribution of organics to their INP activity. Further characterization of the sources and organic and inorganic contributions to terrestrially emitted INPs by comparison to laboratory studies on collected soil dust and plant samples will also be presented.